CA1097725A - Thermal printers - Google Patents
Thermal printersInfo
- Publication number
- CA1097725A CA1097725A CA310,497A CA310497A CA1097725A CA 1097725 A CA1097725 A CA 1097725A CA 310497 A CA310497 A CA 310497A CA 1097725 A CA1097725 A CA 1097725A
- Authority
- CA
- Canada
- Prior art keywords
- thick film
- resistive
- switching element
- printing
- series
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Electronic Switches (AREA)
Abstract
IMPROVEMENTS IN THERMAL PRINTERS
Abstract of the Disclosure Each resistive element of a thermal print bar operated in a scan mode is series connected to a thick film switch which sets a voltage threshold to isolate the element from periodic scan pulses unless coincident with an element selecting data pulse in which case the thick film switches and the resistive element undergoes joule heating.
- i -
Abstract of the Disclosure Each resistive element of a thermal print bar operated in a scan mode is series connected to a thick film switch which sets a voltage threshold to isolate the element from periodic scan pulses unless coincident with an element selecting data pulse in which case the thick film switches and the resistive element undergoes joule heating.
- i -
Description
~Q~7725 This invention relates to thermal print bars for use, for example in facsimile systems.
In a known type of facsimile system, a thermal print bar is moved relative to a receptor of heat sensitive paper.
A surface of the print bar is caused to undergo a localized heating in response to an incoming facsimile signal so an array of pels, or darkened areas, appearing on the heat sensitive paper accurately resemble pictorial or other information contained in the facsimile signal.
A previously proposed form of thermal print bar utilizes an array of elements of resistive material, each element being addressed by a selected pair of control electrodes. The electrodes to the thermal print bar are divided in a regular cartesian arrangement, into scan electrodes and data electrodes.
Scan voltage pulses are applied regularly to successive scan electrodes while data pulses appropriate to an incoming facsimile or like signal are applied to appropriate ones of the data electrodes.
The data pulses may be positive +VD or negative -VD whereas the scan pulses are arranged to be single valued, say vs. Thus when the potential difference across an element of resistive material is a maximum VD + vs, the element should undergo sufficient joule heating that it darkens heat sensitive paper whereas when the potential difference is a minimum, VS - VD~ the resistive element should not undergo sufficient heating for adjacent heat sensitive material to be effected.
The use of a scan system does introduce a problem.
Because of the rate at which the scan pulses are applied, the elements respond to the r.m.s. of the applied voltage v5, so elements although not selected for data printing, receive a voltage higher than VS - VD
and consequently exhibit a measure of joule heating with consequent lQ~7725 ghosting of the heat sensitive paper. This wastes power and can reduce contrast between printed and non-printed parts of a receptor sheet of heat sensitive paper.
The use of discrete diodes in the addressing circuitry has been proposed to overcome the problem. In addition to being expensive, diodes limit the power which can be applied to the resistive heating elements and create reliability problems due to the large number of components needed to be bonded to each substrate.
According to the invention a thermal print bar suitable for use in document reproduction has an array of print elements, each element comprising a series connected heating resistor and switch, said switches each comprising a film of material characterized in a marked heat induced resistivity decrease at a threshold voltage, and means for applying a voltage exceeding said threshold voltage selectively across each of said elements to operate a switch and thereby cause joule heating of its series connected heating resistor.
Preferably the heat sensitive switch is composed of a film of vanadium dioxide (V02). The oxide film can be printed on an appropriate substrate using conventional, inexpensive screen printing techniques.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 shows part-schematically a portion of a thermal print bar; and Figure 2 is a graphical illustration of the current/
voltage relationship for a particular pairing of heat sensitive switch and heating resistor used in the print bar.
Referring specifically to Figure 1, a number of 10~7~25 heating resistor elements 10 are connected into a matrix of column and row electrodes, respectively 11 and 12. The large number of the elements, connected into the electrode matrix in a linear disposition, is used as a thermal print bar, for example, in facsimile systems.
In operation, one component of a facsimile signal is applied successively to column electrodes 11 while other components of the facsimile signal are applied to appropriate ones of the row electrodes 12. According to the total potential difference produced across a resistor element, the element undergoes joule heating and a heat-sensitive receptor sheet drawn across the thermal print bar is printedaccordingly. A scanning pulse is applied cyclically to the electrodes 11 which are termed the scan electrodes, the applied scanning pulses being of fixed amplitude and sense. On the other hand, the electrodes 12 termed the data electrodes, receive as appropriate, a data pulse, the timing of the pulse and the selection of electrode corresponding to a pel, or darkened area to be printed, and being derived through pulse generation circuitry (not shown). The data pulse may be positive or negative according to whether the particular element is required to print or not on the heat sensitive paper. If the scan pulse is at +VS and the data pulse is at VD, then the total potential difference across the resistor element may be VS - VD and the cell characteristics are so chosen that insufficient joule heating occurs in an element to materially darken an area of the receptor sheet with which the particular element is in contact. If the data pulse is at -VD then the total potential difference is v5 + VD and printing on the heat sensitive receptor sheet will occur.
It will be readily appreciated that between the potential differences across the heating resistor of v5 - VD and v5 + VD there will be a range of intermediate voltages at which the heating resistors will in fact undergo some heating. Consequently, $Q97725 when the scanning pulse is applied to a scan electrode without a data pulse being applied to a data electrode, there is still some heating of the resistor elements. Because the scan pulses are applied at a very high rate, the heating resistor responds to the r.m.s. of the scan pulse voltage, the effect being that the heating resistor is, throughout use, in a partially "on" state. This is undesirable, firstly, because power is dissipated and secondly because contrast on the receptor sheet will suffer.
Thus, as shown in Figure 1 between every scan electrode and data electrode there is connected, in series with the resistor element, a heat sensitive thick film switch 16. The switches 16 are printed as films of vanadium dioxide which has been modified as described in U.S. Patent no. 3,542,697 (Chamberland, et al) to give a transition temperature in the range of 40C to 65C and a change in electrical resistivity of two to three orders of magnitude.
Between the temperatures of 40C and 65C, the resistance of a particular sample of the vandium dioxide deposited as a thick film resistor, exhibited a change from 42KQ to lK~.
The thick film switch material may be viewed as having a voltage threshold since when the voltage can generate enough current to produce sufficient joule heating, the switches 16 switch on.
Referring to the l-V characteristics of a single switch 16 in series with heating resistor 10 for a sample of the modified vanadium dioxide thick film switch in the region A below 62V, the I-V curve is essentially linear. In this condition, the switch 16 is off and has a resistance of 42KQ. Together with the heating resistor, chosen to be 4KQ, the total resistance is 46KQ. At about 62V across the circuit, the switch 16 abruptly turns on as shown at B in Figure 2 with its resistance falling to lKQ leaving a circuit resistance of 5KQ. Hence the current changes from 1.3mA
10~725 below 62V to 12mA above 62 volts. In region C the circuit is linear again with a resistance of 5KQ.
When the thermal printing bar of Figure 1 is used as part of a thermal printer operating in a scan or "voltage half-select" mode, a typical required printing voltage is 80V. The corresponding half select voltage is therefore 40V which is below the switching threshold of the thick film switch 16. Without the switch the half selected resistive element would carry lOmA~ With a switch in the off state, it has been shown to carry 0.9mA; the current required is lower by a factor of 10. Also the joule heating (I2R) in the half-selected resistor element is lower by a factor of 100. This permits more thorough heating of selected printer elements in the case of resistance inhomogenities in the heating elements 10.
As described in the previously mentioned Chamberland patent, the thick film switch threshold voltage of the vanadium dioxide film may be altered by combining the material with an oxide of one of the metals ruthenium, iridium, rhodium, rhenium, osmium, indium, thallium or arsenic to give a material with formula - x Mx 4
In a known type of facsimile system, a thermal print bar is moved relative to a receptor of heat sensitive paper.
A surface of the print bar is caused to undergo a localized heating in response to an incoming facsimile signal so an array of pels, or darkened areas, appearing on the heat sensitive paper accurately resemble pictorial or other information contained in the facsimile signal.
A previously proposed form of thermal print bar utilizes an array of elements of resistive material, each element being addressed by a selected pair of control electrodes. The electrodes to the thermal print bar are divided in a regular cartesian arrangement, into scan electrodes and data electrodes.
Scan voltage pulses are applied regularly to successive scan electrodes while data pulses appropriate to an incoming facsimile or like signal are applied to appropriate ones of the data electrodes.
The data pulses may be positive +VD or negative -VD whereas the scan pulses are arranged to be single valued, say vs. Thus when the potential difference across an element of resistive material is a maximum VD + vs, the element should undergo sufficient joule heating that it darkens heat sensitive paper whereas when the potential difference is a minimum, VS - VD~ the resistive element should not undergo sufficient heating for adjacent heat sensitive material to be effected.
The use of a scan system does introduce a problem.
Because of the rate at which the scan pulses are applied, the elements respond to the r.m.s. of the applied voltage v5, so elements although not selected for data printing, receive a voltage higher than VS - VD
and consequently exhibit a measure of joule heating with consequent lQ~7725 ghosting of the heat sensitive paper. This wastes power and can reduce contrast between printed and non-printed parts of a receptor sheet of heat sensitive paper.
The use of discrete diodes in the addressing circuitry has been proposed to overcome the problem. In addition to being expensive, diodes limit the power which can be applied to the resistive heating elements and create reliability problems due to the large number of components needed to be bonded to each substrate.
According to the invention a thermal print bar suitable for use in document reproduction has an array of print elements, each element comprising a series connected heating resistor and switch, said switches each comprising a film of material characterized in a marked heat induced resistivity decrease at a threshold voltage, and means for applying a voltage exceeding said threshold voltage selectively across each of said elements to operate a switch and thereby cause joule heating of its series connected heating resistor.
Preferably the heat sensitive switch is composed of a film of vanadium dioxide (V02). The oxide film can be printed on an appropriate substrate using conventional, inexpensive screen printing techniques.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 shows part-schematically a portion of a thermal print bar; and Figure 2 is a graphical illustration of the current/
voltage relationship for a particular pairing of heat sensitive switch and heating resistor used in the print bar.
Referring specifically to Figure 1, a number of 10~7~25 heating resistor elements 10 are connected into a matrix of column and row electrodes, respectively 11 and 12. The large number of the elements, connected into the electrode matrix in a linear disposition, is used as a thermal print bar, for example, in facsimile systems.
In operation, one component of a facsimile signal is applied successively to column electrodes 11 while other components of the facsimile signal are applied to appropriate ones of the row electrodes 12. According to the total potential difference produced across a resistor element, the element undergoes joule heating and a heat-sensitive receptor sheet drawn across the thermal print bar is printedaccordingly. A scanning pulse is applied cyclically to the electrodes 11 which are termed the scan electrodes, the applied scanning pulses being of fixed amplitude and sense. On the other hand, the electrodes 12 termed the data electrodes, receive as appropriate, a data pulse, the timing of the pulse and the selection of electrode corresponding to a pel, or darkened area to be printed, and being derived through pulse generation circuitry (not shown). The data pulse may be positive or negative according to whether the particular element is required to print or not on the heat sensitive paper. If the scan pulse is at +VS and the data pulse is at VD, then the total potential difference across the resistor element may be VS - VD and the cell characteristics are so chosen that insufficient joule heating occurs in an element to materially darken an area of the receptor sheet with which the particular element is in contact. If the data pulse is at -VD then the total potential difference is v5 + VD and printing on the heat sensitive receptor sheet will occur.
It will be readily appreciated that between the potential differences across the heating resistor of v5 - VD and v5 + VD there will be a range of intermediate voltages at which the heating resistors will in fact undergo some heating. Consequently, $Q97725 when the scanning pulse is applied to a scan electrode without a data pulse being applied to a data electrode, there is still some heating of the resistor elements. Because the scan pulses are applied at a very high rate, the heating resistor responds to the r.m.s. of the scan pulse voltage, the effect being that the heating resistor is, throughout use, in a partially "on" state. This is undesirable, firstly, because power is dissipated and secondly because contrast on the receptor sheet will suffer.
Thus, as shown in Figure 1 between every scan electrode and data electrode there is connected, in series with the resistor element, a heat sensitive thick film switch 16. The switches 16 are printed as films of vanadium dioxide which has been modified as described in U.S. Patent no. 3,542,697 (Chamberland, et al) to give a transition temperature in the range of 40C to 65C and a change in electrical resistivity of two to three orders of magnitude.
Between the temperatures of 40C and 65C, the resistance of a particular sample of the vandium dioxide deposited as a thick film resistor, exhibited a change from 42KQ to lK~.
The thick film switch material may be viewed as having a voltage threshold since when the voltage can generate enough current to produce sufficient joule heating, the switches 16 switch on.
Referring to the l-V characteristics of a single switch 16 in series with heating resistor 10 for a sample of the modified vanadium dioxide thick film switch in the region A below 62V, the I-V curve is essentially linear. In this condition, the switch 16 is off and has a resistance of 42KQ. Together with the heating resistor, chosen to be 4KQ, the total resistance is 46KQ. At about 62V across the circuit, the switch 16 abruptly turns on as shown at B in Figure 2 with its resistance falling to lKQ leaving a circuit resistance of 5KQ. Hence the current changes from 1.3mA
10~725 below 62V to 12mA above 62 volts. In region C the circuit is linear again with a resistance of 5KQ.
When the thermal printing bar of Figure 1 is used as part of a thermal printer operating in a scan or "voltage half-select" mode, a typical required printing voltage is 80V. The corresponding half select voltage is therefore 40V which is below the switching threshold of the thick film switch 16. Without the switch the half selected resistive element would carry lOmA~ With a switch in the off state, it has been shown to carry 0.9mA; the current required is lower by a factor of 10. Also the joule heating (I2R) in the half-selected resistor element is lower by a factor of 100. This permits more thorough heating of selected printer elements in the case of resistance inhomogenities in the heating elements 10.
As described in the previously mentioned Chamberland patent, the thick film switch threshold voltage of the vanadium dioxide film may be altered by combining the material with an oxide of one of the metals ruthenium, iridium, rhodium, rhenium, osmium, indium, thallium or arsenic to give a material with formula - x Mx 4
Claims (3)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-
1. A thermal printer for printing on a heat-sensitive receptor, the printer comprising:
an array of thick film resistive printing elements and, series-connected to each resistive print element, a respective switching element, each switching element comprising a thick film deposit of a material undergoing a marked, reversible resistivity change at a predetermined temperature owing to change in crystalline structure of said material; and addressing circuitry for selectively applying a potential difference across a combination of a printing element and its series-connected switching element thereby to cause joule heating of said switching element to promote a reduction in the material resistivity whereupon the series-connected resistive printing element experiences a corresponding increase in potential difference and consequently undergoes joule heating.
an array of thick film resistive printing elements and, series-connected to each resistive print element, a respective switching element, each switching element comprising a thick film deposit of a material undergoing a marked, reversible resistivity change at a predetermined temperature owing to change in crystalline structure of said material; and addressing circuitry for selectively applying a potential difference across a combination of a printing element and its series-connected switching element thereby to cause joule heating of said switching element to promote a reduction in the material resistivity whereupon the series-connected resistive printing element experiences a corresponding increase in potential difference and consequently undergoes joule heating.
2. A thermal print bar as claimed in claim 1 in which said switch material incorporates vanadium dioxide.
3. A thermal print bar as claimed in claim 1 in which the resistive material is thick film printed as a single elongate strip.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA310,497A CA1097725A (en) | 1978-09-01 | 1978-09-01 | Thermal printers |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA310,497A CA1097725A (en) | 1978-09-01 | 1978-09-01 | Thermal printers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1097725A true CA1097725A (en) | 1981-03-17 |
Family
ID=4112271
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA310,497A Expired CA1097725A (en) | 1978-09-01 | 1978-09-01 | Thermal printers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1097725A (en) |
-
1978
- 1978-09-01 CA CA310,497A patent/CA1097725A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |